The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes using drilling equipment situated at surface and a drill string extending from equipment on the surface to a subterranean zone of interest such as a formation. The drill string can extend thousands of meters below the surface. The downhole terminal end of the drill string includes a drill bit for drilling the wellbore. Drilling wellbores also typically involves using some sort of drilling fluid system to pump a drilling fluid (“mud”) through the inside of the drill string, which cools and lubricates the drill bit and then exits out of the drill bit and carries rock cuttings back to the surface. The mud also helps control bottom hole pressure and prevents hydrocarbon influx from the formation into the wellbore and potential blow out at the surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or to follow a prescribed path. At the downhole terminal end of the drill string is a bottom-hole-assembly (“BHA”) that includes 1) the drill bit; 2) a steerable downhole mud motor; 3) sensors including survey equipment (e.g. one or both of logging-while-drilling (“LWD”) and measurement-while-drilling (“MWD”) tools (both “LWD” and “MWD” are hereinafter collectively referred to as “MWD” for simplicity)) to evaluate downhole conditions as drilling progresses; 4) telemetry equipment to transmit data to surface; and 5) other control equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars known as drill pipe. The MWD equipment is used to provide in a near real-time mode downhole sensor and status information to the surface while drilling. This information is used by the rig operator to make decisions about controlling and steering the drill string to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, hydrocarbon size and location, etc. This can include making intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real-time data allows for a relatively more economical and more efficient drilling operation.
MWD is performed using MWD tools, each of which contains a sensor package to survey the wellbore and to send data back to the surface by various telemetry methods. Such telemetry methods include, but are not limited to telemetry via a hardwired drill pipe, acoustic telemetry, telemetry via a fiber optic cable, mud pulse (“MP”) telemetry and electromagnetic (“EM”) telemetry.
MP telemetry involves using a fluid pressure pulse generator to create pressure waves in the circulating mud in the drill string. Mud is circulated between the surface and downhole using positive displacement pumps. The resulting flow rate of mud is typically constant. The pulse generator creates pressure pulses by changing one or both of the flow area and path of the mud as it passes through the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the drilling fluid. The pressure differentials or pulses may either be negative pulse or positive pulses in nature. Valves that use a controlled restriction within the circulating mud stream create a positive pressure pulse. Some valves are hydraulically powered to reduce the required actuation power typically by using a main valve controlled by a pilot valve. The pilot valve closes a flow restriction, which actuates the main valve and creates a change in pressure.
The pressure pulses generated by the pulse generator are used to transmit information acquired by the downhole sensors. Signals from the sensors are received and processed in a data encoder in the BHA where the data is digitally encoded. A controller then actuates the pulse generator to generate the mud pulses, which are modulated to represent the data. For example, the directional or inclination data is conveyed or modulated using the physical mud pulse by generating the mud pulse at a particular amplitude and frequency. Typically a high-frequency sinusoid waveform is used as a carrier signal, but a square wave pulse train may also be used.
A typical arrangement for EM telemetry uses parts of the drill string as an antenna. The drill string is divided into two conductive sections by including an electrically insulating joint or connector (a “gap sub”) in the drill string. The gap sub is typically placed within the BHA such that metallic drill pipe in the drill string above the gap sub serves as one antenna element and metallic sections below the gap sub serve as another antenna element. EM telemetry signals can then be transmitted by applying electrical signals across the two antenna elements. The signals typically include very low frequency AC signals applied in a manner that codes information for transmission to the surface. The electromagnetic signals may be detected at the surface, for example by measuring electrical potential differences between the drill string and one or more grounding rods spaced from the drill string.
Both EM and MP telemetry systems use a downhole source of power. One common power source is downhole batteries.
MWD systems contain power systems that are generally of two types. The first type uses a turbine or other generator to produce power downhole, and the second type uses specialized batteries developed for downhole applications. Turbines are powered via circulation of drilling fluid, whereas batteries operate independently of drilling fluid flow. In some cases, both types of power systems are used to help ensure adequate power is delivered to service all downhole load requirements. The batteries are typically lithium-thionyl chloride batteries, which provide high energy density and can withstand temperatures of up to approximately 180-200° C. Many downhole batteries are rated to be able to store approximately 26-28 A·h @ 3.6 V per cell. The load is generally determined by electrical components within the BHA, drill collar geometry, gap sub or mud pulser specifications, and the properties of the surrounding formation. As an example, the current drawdown on the gap sub will vary depending on signal attenuation to the surface; or in a mud pulser, the current drawdown will vary with the torque required to actuate the valve that generates mud pulses. An example of a typical industry battery is Exium™ Technologies Inc. MWD 3.6 DD size Li-SOCL2.
Referring now to FIGS. 1(a) and (b), there are shown prior art battery management systems for use in downhole applications. FIG. 1(a) shows a first battery 100a and a second battery 100b (collectively, the first and second batteries 100a,b are the “batteries 100”) electrically connected in parallel via power lines 104. The power lines 104 terminate at a power bus 102 to which an electrical load, such as a motor when MP telemetry is used or the gap sub when EM telemetry is used, is connected. The system of FIG. 1(a) includes no switching circuitry and no data collection circuitry; that is, both of the batteries 100 are connected constantly to the power bus 102, and neither the voltages of the batteries nor the currents output by the batteries are readily determinable.
FIG. 1(b) shows another exemplary prior art battery management system. In FIG. 1(b), the first and second batteries 100a,b are electrically connected in parallel as they are in FIG. 1(a), and the batteries 100 are also electrically connected to a combined power and data bus 106 via the power lines 104. A first circuit board 110a is placed adjacent the first battery 100a and a second circuit board 110b is placed adjacent the second battery 100b (collectively, the circuit boards 110a,b are the “circuit boards 100”), and on each of the circuit boards 110 is switch and measurement circuitry that measure voltage and current and calculate the power used by the system. The circuit boards 110 are communicative with the power and data bus 106 via control and data lines 108, over which control signals sent to the circuit boards 110 and data signals received from the circuit boards 110 are multiplexed.
Notwithstanding these existing battery management systems, there exists a continued need for methods, systems, and techniques to manage batteries used in downhole MWD applications.